@mattie g is completely right about the time of the year. As far as vegetation and snow weenies go, Nov/Dec is the best time for it to be dry.
Interestingly, both IAD and BWI have now slipped below normal on precip for the year, IAD significantly so (close to -8”). DCA is still above due to the summer convection favoring areas near the beltway and east. On a multi-year basis we are still just fine, but something to keep an eye on.